1. Field of the Invention
The present invention relates to a bi-phase mark reproduction apparatus which can correct an error produced in bi-phase marks on the basis of regularity of the bi-phase marks so that an error rate of demodulated data can be reduced by half and an optical disk drive device equipped with the same.
2. Description of the Related Art
Heretofore, an optical disk such as CD-R and CD-RW is provided with guide grooves named pre-grooves so as to form tracks upon recording of data. The grooves meander very slightly in the radial direction thereof with a meandering amount of about ±0.03 μm and are named wobbles. The wobbles are FM modulated in 22.05 kHz±1 kHz when a medium is reproduced at a standard speed. That is, data FM-modulated with frequency shift of ±1 kHz is added onto a carrier of 22.05 kHz.
The data recorded by the FM modulation includes absolute time information (hereinafter referred to as address information) recorded on the medium. This information is named ATIP (Absolute Time In Pre-grooves). The ATIP includes control signals such as a maximum recordable time and laser power setting information upon recording, recommended to the medium in addition to the address information.
FIG. 11 shows a frame format of the ATIP. The ATIP data constitutes the frame shown in FIG. 11. The frame format includes “Sync” of a synchronous code, “Minutes”, “Seconds” and “frames” constituting time information and “CRC remainder” for error detection. The “Minutes”, “Seconds” and “frames” constituting the time information are sometimes represented as “data portion” collectively as information for decision of an address. FIG. 12 shows the bi-phase mark signal and the ATIP data when a value just before the synchronous code is “0” and FIG. 13 shows the bi-phase mark signal and the ATIP data when a value just before the synchronous code is “1”. The bi-phase mark signal is subjected to the bi-phase mark modulation as shown in FIGS. 12 and 13 and then recorded (cut in an original disk) in the medium.
The bi-phase mark modulation is now described. Referring to FIG. 11, in the frame format of the ATIP, 4 bits are assigned to the “Sync” of the synchronous code, 8 bits to the “Minutes”, 8 bits to the “Seconds” and 8 bits to the “Frames” constituting the time information and 14 bits to the “CRC remainder” for the error detection as data. The ATIP data is subjected to the bi-phase mark modulation.
As shown in FIGS. 12 and 13, the bi-phase mark modulation is to convert data bits into data for channel bits by double bit clocks. For example, 4 bits are converted into 8 bits. In the modulation, when bit data before conversion is “0”, the channel bit data is assigned any of “00” and “11” by selecting it so that reversal of bits in the channel bit data is made before and behind each partition for every two bits. On the other hand, when bit data before conversion is “1”, the channel bit data is assigned any of “10” and “01” so that reversal of bits in the channel bit data is made before and behind each partition for every two bits. Accordingly, one-bit data before conversion is converted into the channel bit data so that reversal of bits in the channel bit data is made before and behind each partition for every two bits.
Whether bit data of “0” before conversion is converted into any of “00” or “11” and whether bit data of “1” before conversion is converted into any of “10” or “01” are decided depending on whether a bit produced just before in the channel bit data is “0” or “1”. For example, when the “Minutes” is “0110”, it is converted into “00101011” or “11010100”. This conversion is decided depending on whether a bit produced just before the “Minutes” is “0” or “1”. When the bit is “0”, the “Minutes” is converted into “11010100” and when the bit is “1”, the “Minutes” is converted into “00101011”.
The channel bit data is recorded as wobble as shown by the wave form of FIGS. 12 and 13. The ATIP information read out from an optical disk is subjected to error detection by means of CRC of the “CRC remainder” and when there is no error, the ATIP information is utilized.
In reproduction of the ATIP (bi-phase mark), information is sometimes lost due to stain or flaw on the optical disk or disturbance in focusing, tracking or control of a spindle or the like. At this time, in an optical disk drive device such as CD-R and CD-RW, since the control of the spindle upon writing uses a period of the synchronous signal in the ATIP as a fundamental wave, it is necessary to avoid disappearance of synchronization or taking-in of wrong data when the bi-phase mark information is lost during writing.
FIG. 14 is a block diagram schematically illustrating a conventional bi-phase mark reproduction apparatus. In FIG. 14, numeral 1101 represents a bi-phase mark input signal read out from an optical disk, 1102 a synchronism detection unit which extracts a synchronous code from the bi-phase mark input signal 1101 and produces a signal taking-out timing, 1103 a clock reproduction unit which reproduces a clock signal on the basis of a signal edge in the bi-phase mark signal 1101 by means of PLL, and 1104 a data separating signal which is produced by the synchronism detection unit 1102 to separate the synchronous code portion (“Sync” of FIG. 11), the data portion (“Minutes”, “Seconds” and “frames” of FIG. 11) and the error detection code portion (“CRC remainder” of FIG. 11).
Numeral 1105 represents a data conversion unit which separates the synchronous code portion from the bi-phase mark input signal 1101 by the data separating signal 1104 and demodulates data of the data portion and the error detection code portion, 1106 demodulated data which is produced by the data conversion unit 1105 after being demodulated by the data conversion unit 1105, and 1107 an error detection circuit which subjects the data 1106 demodulated by the data conversion unit 1105 to the error detection by means of CRC. As a result of the error detection, an error information output 1108 indicates whether the demodulated data 1106 is valid (error is not contained) or invalid (error is contained).
In reproduction, the bi-phase mark signal read out from the optical disk such as CD-R and CD-RW is synchronized with the clock signal from the clock reproduction unit 1103 in the synchronism detection unit 1102 and the data conversion unit 1105 to be converted into the channel bit data. The synchronism detection unit 1102 detects 8-bit synchronous code (11101000 and 00010111) shown in FIGS. 12 and 13 from the channel bit data and produces the data separating signal 1104.
In the data conversion unit 1105, the channel bit data left by removing the synchronous code in accordance with the data separating signal 1104 is demodulated into the data bits each constituting one bit for every two bits. The demodulation is made in accordance with the rule that when the channel bit data of two bits is “00” or “11”, it is demodulated into “0” and when it is “01” or “10”, it is demodulated into “1”. At this time, as described above, the channel bit data is necessarily reversed before and behind the partition of the demodulation unit of two bits and the synchronous code (that is, “0” is reversed to “1” and “1” is reversed to “0”).
As described above, the data 1106 demodulated by the data conversion unit 1105 is subjected to the error detection using CRC in the error detection circuit 1108 to judge whether the demodulated data 1106 contains no error and is valid or contains any error and is invalid. Only the demodulated data 1106 judged to be valid is used as the ATIP information.
When the ATIP information is read out from the optical disk, the bi-phase mark signal is sometimes lost or disturbed due to stain or flaw on the optical disk or disturbance in focusing, tracking or control of a spindle or the like. In this case, in the conventional processing described above, when the bi-phase mark signal is lost or disturbed, the signal is converted into wrong channel bit data and accordingly error is sometimes produced in the demodulated data. Further, the wrong demodulated data is distinguished by the error detection using CRC and is judged to be invalid and accordingly there is a problem that the ATIP information for the wrong demodulated data portion is not obtained. When the ATIP information is not obtained continuously upon writing, it sometimes happens that the control of spindle is impossible and the writing is stopped.